The present invention relates to an apparatus for use in fusion refining electrolysis. The apparatus includes a carbon electrode surrounded by a self-supporting, gastight, and temperature stable protective globe.
In fusion refining electrolysis, such as, for example, three-layer electrolysis for refining aluminum as cathodes, carbon electrodes which are used are usually of graphitized carbon. These electrodes are immersed directly in the molten cathode metal. The carbon material vitrifies above the upper surface of the molten bath very vigorously, because of the high temperature of the electrodes and the unblocked admission of atmospheric oxygen.
As a consequence, the cross-section of the electrode can be so greatly reduced that the under part of the electrode breaks off. This leads to a high carbon material consumption of about 8% with respect to the amount of metal produced. In order to reduce this high carbon material consumption, the admission of atmospheric oxygen must be stopped. Up to now, various methods have been proposed.
Through impregnation of the carbon electrodes, for example, with borax or phosphates, the carbon material consumption can be reduced to about 4%. In this case, the cathode metal becomes contaminated by the impregnation agent.
Coating or casting around the carbon electrode with already refined aluminum offers inadequate protection against oxygen. The aluminum can melt away at the given temperatures of the electrode upper surface so that the carbon material burns off beneath the protective layer.
As another proposal, the carbon electrodes are given a several mm thick ceramic layer, for example, by means of a plasma spray. Unfortunately, the different thermal expansion rates of carbon and ceramic leads to thermal stress and thereby deterioration of the ceramic layer.
It would, therefore, be desirable to protect a carbon electrode effectively to endure despite oxygen admission so that the consumption of the carbon material is at most 1% and no contamination enters the cathode metal. The loss in carbon material due to consumption is related to the produced amount of refined aluminum, subject to burning losses and losses from break off of parts of the electrodes.